In order to monitor a process, and thereby to control it, it is necessary to identify parameters which reliably reflect the state of the process, and which can be conveniently measured on a regular basis. In the case of a photographic process, it is common to illustrate the photographic response of a particular film, following development in that process, by a curve, sometimes called the "characteristic curve" for the material being measured, representing the relationship between developed density and the logarithm of exposure. This curve is often referred to as the H & D curve, after Hurter and Driffield, The Journal of the Society of Chemical Industry, No. 5, Vol. IX, May 31, 1890.
The "characteristic curve" is determined using a control strip as is well known in the art. The control strip is produced by taking a small piece of film and exposing it in a sensitometer by contact with an original step wedge, which has, typically, 21 densities in steps of 0.15log exposure units (for X-ray films, for example), with light of a colour appropriate to the type of film being used for process control (typically either blue or green for X-ray films). The exposed strip is processed in the processor whose performance is being monitored, and is then ready to be measured.
Densities measured on the control strip are plotted against the relative log exposure, and important process control parameters from the resulting curve can be obtained which characterize the state of the process.
However, the process control parameters which are currently in use, almost universally, are not adequate descriptors of the response of the system, and in each case are themselves dependent on unmeasured variables in the system.
Moreover, in process control systems, for example the KODAK `X-Omat` Process Control Manager, where the variability of the parameters is linked to a system for diagnosing the process, the diagnosis is limited when the variables cannot properly be separated.
It is therefore an object of the present invention to provide an improved process control system for automatic photographic processing machines.
It is a further object of the present invention to produce photographic materials based on the use of mutually exclusive control parameters which are more usefully related to the shape of the characteristic curve for the material in question.